The invention relates to a method for fastening a printing plate onto a plate cylinder of a rotary printing machine, in particular a web-fed rotary offset printing machine. The invention also relates to a device for fastening a printing plate to a plate cylinder of a rotary printing machine, in particular a web-fed rotary offset printing machine. In the heretofore known method and device of these general types, provision is made for the printing plate to have a first end and a second end, which are accomodated in a clamping gap formed in the circumferential surface of the plate cylinder, at least approximately parallel to the axis of rotation of the plate cylinder.
Before the printing plate can be fastened to the plate cylinder, both of the ends of the printing plate are initially bent over with the aid of an edge bending device located outside the printing machine. The bent-over first end is then inserted into the clamping gap and held firmly with the aid of a clamping device. The printing plate is then wound onto the circumferential surface of the plate cylinder until the bent-over second end is located over the clamping gap. Finally, this bent-over second end is likewise inserted into the clamping gap and held firmly with the aid of a second clamping device. Each clamping device includes at least one movable jaw which, in order to hold the bent-over end firmly, can be set against the latter and forces it against a side wall of the clamping gap. The clamping gap, therefore, has to be of sufficient width for accommodating the clamping devices. In addition, the clamping devices increase the construction expense and, therefore, the production and maintenance costs of the plate cylinder. In the case of rotary printing machines, however, it is desired that the clamping gap be as narrow as possible, because the part of the circumferential surface of the plate cylinder which is occupied by the clamping gap is not available for the printing. Primarily, for web-fed rotary printing machines wherein paper is infed in the form of a paper web unwound from a paper reel, the width of the clamping gap is a measure of the paper wastage.
A further disadvantage of the heretofore known method and device is that the edge-bending device has to be procured and operated separate from the actual printing machine. In addition, the printing plates provided with bent-over ends present problems when stored in a magazine for automatic printing-plate changing, because at least two printing plates have to be stored or handled simultaneously in such a magazine, so that damage may possibly occur quite easily to the sensitive printing surface of one printing plate by the bent-over ends of the other printing plate.
Furthermore, the bending over of the edges must be carried out very carefully, because the printing plates cannot be aligned correctly on the plate cylinder if the required angles and radii are not respected when the edges are bent over, and the edge-bending line does not extend perpendicularly to the longitudinal edges of the printing plate. Even a slight shape difference between printing plate and cylinder increases the risk that the printing plate will tear on the cylinder, as a result of which the entire printing machine can be damaged seriously.
The published European Patent Document EP 0 654 349 B2 discloses a sheet-fed rotary printing machine having an automatic plate changing device, wherein the printing plates are fed in flat form at least approximately tangentially to the plate changing device from a magazine and, with the aid of an edge turning or bending bar and a pressure roller, are provided with a bent-over edge on one side immediately before being inserted into the appertaining plate clamping device. In the described device, both the plate leading edge and the plate trailing edge are held in separate clamping devices, which can be operated separately and are spaced at a great circumferential distance from one another, the great spaced distance being usual for sheet-fed printing machines.
It is therefore an object of the invention to provide a method and a device of the type mentioned at the introduction hereto wherein the plate cylinder is constructed more simply, the clamping gap is narrower and a separate edge-turning or bending device is unnecessary.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method for fastening a printing plate to a plate cylinder of a rotary printing machine, more particularly a web-fed rotary offset printing machine, the printing plate having a first end and a second end, both of which being accommodatable in a clamping gap formed in the circumferential surface of the plate cylinder at least approximately parallel to the axis of rotation of the plate cylinder, which comprises the following method steps: inserting the first end of the printing plate into the clamping gap; bending over an edge of the printing plate in the region of the first end at an edge of the clamping gap; winding the printing plate onto the circumferential surface of the plate cylinder until the second end of the printing plate comes to lie over the clamping gap; and pressing the second end of the printing plate into the clamping gap, and deforming the printing plate so that the second end blocks the first end of the printing plate in the clamping gap.
In accordance with another mode, the method includes, in order to bend over an edge, setting an edge-bending element against the plate cylinder in the region of the trailing edge of the clamping gap, as viewed in the continuous printing direction, and rotating the plate cylinder in the continuous printing direction relative to the edge-bending element.
In accordance with a further mode, the method includes providing a pressure roller as the edge-bending element.
In accordance with an added mode, the method further includes, in the winding step, setting a pressure element against the printing plate, and rotating the plate cylinder in the continuous printing direction relative to the pressure element.
In accordance with an additional mode, the method further includes providing a pressure roller as the pressure element.
In accordance with yet another mode, the method further includes providing the pressure roller as a single pressure roller serving as both the edge-bending element and a pressure element and, in the winding step, setting the pressure roller against the printing plate, and rotating the plate cylinder in the continuous printing direction relative to the pressure roller.
In accordance with yet a further mode, the method further includes setting a blade against the second end of the printing plate for inwardly pressing the printing plate, and pressing the second end of the printing plate into the clamping gap.
In accordance with yet an added mode, the method further includes providing for the trailing side wall of the clamping gap, as viewed in the continuous printing direction, to extend at an acute angle to the circumferential surface of the plate cylinder.
In accordance with yet an additional mode, the method further includes providing the clamping gap with a cross section having at least approximately the shape of a trapezoid, with the short base thereof forming the opening of the clamping gap.
In accordance with still another mode, the method further includes introducing compressed air into the clamping gap in order to loosen the printing plate from the plate cylinder.
In accordance with still a further mode, the method further includes assigning a blanket cylinder to the plate cylinder, the blanket cylinder bearing a rubber blanket sleeve which is pullable axially off the blanket cylinder.
In accordance with another aspect of the invention, there is provided a device for fastening a printing plate to a plate cylinder of a rotary printing machine, more particularly a web-fed rotary offset printing machine, the printing plate having a first end and a second end, both of which are accommodatable in a clamping gap formed in the circumferential surface of the plate cylinder, at least approximately parallel to the axis of rotation of the plate cylinder, comprising an edge-bending device serving for bending an edge of the printing plate in a region of a first end thereof on an edge of the clamping gap, after the first end has been inserted into the clamping gap; a winding device serving for winding the printing plate onto the circumferential surface of the plate cylinder, due to which a second end of the printing plate has come to lie over the clamping gap; and a pressing device serving for pressing the second end of the printing plate into the clamping gap, and for deforming the second end, so that the second end blocks the first end of the printing plate in the clamping gap.
In accordance with a further feature of the invention, the edge-bending device includes an edge-bending element and is formed so that during the edge bending, the edge-bending element is pressed against the plate cylinder in the region of the trailing edge of the clamping gap, as viewed in the continuous printing direction.
In accordance with an added feature of the invention, the edge-bending element is formed by a pressure roller.
In accordance with an additional feature of the invention, the winding device includes a pressure element and is formed so that the pressure element is set against the plate cylinder during the winding.
In accordance with yet another feature of the invention, the pressure element is formed by a pressure roller.
In accordance with yet a further feature of the invention, the pressure roller is a single pressure roller serving both as the edge-bending element and as a pressure element and, in the winding step, the pressure roller being set against the printing plate, and the plate cylinder being rotated in the continuous printing direction relative to the pressure roller.
In accordance with yet an added feature of the invention, the pressing device includes a blade and is formed so that during the inward pressing, the blade is set against the second end and presses the second end into the clamping gap.
In accordance with yet an additional feature of the invention, the trailing side wall of the clamping gap, as viewed in the continuous printing direction, extends at an acute angle to the circumferential surface of the plate cylinder.
In accordance with still another feature of the invention, the clamping gap has a cross section at least approximately having the form of a trapezoid, with the short base thereof forming the opening of the clamping gap.
In accordance with still a further feature of the invention, the fastening device further comprises a compressed-air line connected to the clamping gap, via which the clamping gap is actable upon by compressed air in order to loosen the printing plate from the plate cylinder.
In accordance with a concomitant feature of the invention, the fastening device further comprises a blanket cylinder assigned to the plate cylinder, the blanket cylinder bearing a rubber blanket sleeve pullable axially from the blanket cylinder.
Accordingly, the method according to the invention comprises the following method steps: inserting the first end of the printing plate into the clamping gap; bending or turning over the edge of the printing plate in the region of a first end thereof at an edge of the clamping gap; winding the printing plate onto the circumferential surface of the plate cylinder until a second end of the printing plate comes to lie over the clamping gap; and pressing the second end of the printing plate into the clamping gap, and deforming the printing plate, so that the second end blocks the first end of the printing plate in the clamping gap.
Furthermore, the fastening device according to the invention comprises the following features: an edge-bending device serving for bending over an edge of the printing plate in the region of the first end thereof on an edge of the clamping gap, after the first end has been inserted into the clamping gap; a winding device serving for winding the printing plate onto the circumferential surface of the plate cylinder, due to which the second end of the printing plate comes to lie over the clamping gap; and a pressing device serving for pressing the second end of the printing plate into the clamping gap, and deforming the second end, so that the second end blocks the first end of the printing plate in the clamping gap.
Because the printing plate, in the case of this method and this device, is initially inserted by the first end thereof into the clamping gap, and the edge at the first end is then bent or turned over at an edge of the clamping gap, a separate edge-bending or turning device, i.e., one that is separate from the printing machine, and the operations associated therewith are unnecessary; and it is possible for flat printing plates, i.e., those without bent-over ends, to be used, which can be handled and stored easily, in a space-saving and reliable or safe manner. Because, in addition, the action of bending over the edge of the printing plate is carried out at one edge of the clamping gap, i.e., in direct cooperation with the outer contour of the plate cylinder, a high accuracy of fit is achieved between printing plate and plate cylinder in the region of the bent-over end, and the printing plate is aligned exactly with the plate cylinder in the region of the first end thereof.
Because, furthermore, both ends are blocked in the clamping gap, after the second end of the printing plate has been introduced, clamping devices are no longer required. The method may provide that, for the purpose of bending over an edge, an edge-bending element is set against the plate cylinder in the region of the trailing edge of the clamping gap, as viewed in the continuous printing direction, and the plate cylinder is rotated in the continuous printing direction relative to the edge-bending element. The relative rotation between plate cylinder and edge-bending element can be effected by rotating the plate cylinder in the continuous printing direction and/or rotating the edge-bending element counter to the continuous printing direction.
In a corresponding manner, in the fastening device of the invention, provision may be made for the edge-bending device to include an edge-bending element and be formed so that, as the edges are being bent over, the edge-bending element presses against the plate cylinder in the region of the trailing edge of the clamping gap, as viewed in the continuous printing direction.
In the method and the device, provision may be made for the edge-turning element to be formed as a pressure roller.
In addition, for the purpose of winding, provision may be made in the method for a pressure element to be set against the printing plate and for the plate cylinder to be rotated in the continuous printing direction relative to the pressure element. The relative rotation between the plate cylinder and the pressure element may be effected by rotating the plate cylinder in the continuous printing direction and/or rotating the pressure element counter to the continuous printing direction.
In a corresponding manner, provision may be made in the fastening device of the invention for the winding device to comprise a pressure element and to be formed in a manner that, during the winding operation, the pressure element is set against the plate cylinder.
In the method and the device, provision may be made for the pressure element to be formed as a pressure roller.
Furthermore, in the method and the device, provision may be made for both the edge-bending element and the pressure element to be formed by a single pressure roller. In this way, the expenditure for construction is further reduced.
Furthermore, for the purpose of inward pressing, provision may be made in the method for a blade to be set against the second end of the printing plate and to press the latter into the clamping gap. If the blade is pressed further into the clamping gap, after it has been set against the second end of the printing plate, the second end will yield to this action by being deformed and wrapped with an at least approximately U-shaped cross section around the cutting edge of the blade, so that the edge contacts the bent-over first end which is already located in the clamping gap. The blade is withdrawn from the clamping gap when the edge of the second end, now bent in a U-shape, is supported on the bent-over first end. As a result, both ends of the printing plate are blocked in the clamping gap.
In a corresponding manner, provision can be made in the fastening device according to the invention for the pressing device to comprise a blade and to be formed in a way that, during the inward pressing, the blade is set against the second end and presses the latter into the clamping gap.
Furthermore, provision can be made in the method and the fastening device for the trailing side wall of the clamping gap, as viewed in the continuous printing direction, to extend at an acute angle to the circumferential surface of the plate cylinder. The front side wall, therefore, springs back under the circumferential surface in the manner of an undercut. If, then, the second end of the printing plate is deformed as it is pressed into the clamping gap, and is supported on the leading side wall and the opposite trailing side wall of the clamping gap, as viewed in the continuous printing direction, the leading side wall better absorbs the radially outwardly directed centrifugal forces, which are produced during the rotation of the plate cylinder during the printing operation and attempt to drive both ends of the printing plate out of the clamping gap, than if it were to extend perpendicularly to the circumferential surface of the plate cylinder, i.e., radially.
Furthermore, provision may be made for the method and the fastening device according to the invention for the cross section of the clamping gap to have the shape at least approximately of a trapezoid, the short base of which forms the opening of the clamping gap.
Also, provision may be made in the method for compressed air to be fed into the clamping gap in order to loosen the printing plate from the plate cylinder. The compressed air forces the deformed second end of the printing plate out of the clamping gap, so that the printing plate can be unwound from the plate cylinder and the bent-over first end of the plate can be removed from the clamping gap.
In a corresponding manner, provision may be made in the fastening device according to the invention for a compressed-air line to be connected to the clamping gap, via which the clamping gap is actable upon by compressed air in order to loosen the printing plate from the plate cylinder.
In addition, provision may be made in the method and the fastening device for the plate cylinder to have a blanket cylinder assigned thereto, the blanket cylinder bearing a rubber blanket sleeve which is pullable axially off the blanket cylinder. Particularly good printing results can be achieved with such a rubber blanket sleeve, because it is endless, as viewed in the circumferential direction, i.e., it has no gap and rests on the blanket cylinder uniformly even at high rotation speeds. In addition, the down times of the printing machine, which are caused by a damaged rubber blanket or a web break, during which the paper web winds around the blanket cylinder, are reduced considerably, because the affected rubber blanket sleeve with the section web wound around it can simply be removed axially from the blanket cylinder through an opening formed in the side wall of the printing machine and can then be replaced by a new rubber blanket sleeve. The result is that the removal of the compressed and hardened layer of paper web and ink from the blanket cylinder, which is required when conventional finite-length rubber blankets are used and is very time-consuming, is thus dispensed with.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for fastening a printing plate to a plate cylinder of a rotary printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: